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IV Characteristics of a filament light bulb
Introduction
This experiment plots the IV characteristics of a lightbulb; that is, you can plot how the current changes when you change the voltage (the I is for current and the V is for voltage). This depends on the resistance of the bulb, which changes depending on the voltage. Effectively, the bulb gets hotter and hotter as the voltage increases, until it is eventually white-hot. As the temperature of the metal filament gets hotter, its resistance changes. As long ago as 1802, Humphry Davy demonstrated a filament light at the Royal Institution. However, it is usually Thomas Edison who gets the credit much later (he wasn’t born until 1847!), as he was able to come up with an entire practical system for electric lighting. Edison was a prolific inventor and held over one thousand patents.
The objective
To plot the relationship between the current and the voltage for a filament lightbulb.
The apparatus
You will need:
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A DC power supply
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Two multimeters
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A breadboard;
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A 12V filament lightbulb
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A 4.7k ohm variable resistor
The variables
The independent variable is the voltage, and the dependent variable is the current. You are effectively controlling the voltage across the diode with the potentiometer.
The circuit
The circuit for measuring IV characteristics for any device is mostly the same. A power source is connected in series with a variable resistor, an ammeter, and the device being tested. A voltmeter is connected across the device, which in this case is the filament light bulb.
The method
Switch on the power. With the power on, you now need to vary the resistance of the variable resistor (sometimes called a potentiometer or even just a pot); this will change the current flowing through the device.
Take readings at every 0.5 volt up to 12V.
The Video
Watch a video of the IV Characteristics e-practical here.
This shows how to use it and how to collect the data.
The E-Practical
Perform the experiment yourself, collect your own data, make mistakes and be able to correct them. The e-practical requires that your browser canrun WebGL 2 (usually found on Windows browsers, safari on iOS, and various Mobile browsers, test with https://get.webgl.org/webgl2/). This link is for students and evaluation only, schools should purchase a site licence.
All the e-practicals will run on devices as small as a mobile 'phone. However, the best experience is on a PC using a mouse which gives very precise control, but if you are limited to a small device, consider using a stylus or a blue tooth mouse.
On a portable device, make sure you click on 'Toggle onscreen controls'. The left joystick controls movement, the right joystick controls direction and where you are looking.
In the e-practical you can switch on the power supply by clicking on the red on/off button. You should see the Output indicator on the power supply screen indicate a connection,
The variable resistor can be varied by placing the mouse cursor over the knob and using the mouse wheel or by dragging if using the on-screen controls. Note that the current is displayed on the right-hand meter and is displayed in milliamps.
The Results
Plot a graph of current in amps on the Y-axis and volts on the X-axis for the entire range of voltage readings. What you should see is the voltage and amperage both increasing together, with the current tailing off as the filament gets hotter and more resistant.
Further Discussion
What is the physical reason that explains why the resistance of the metal filament increases with temperature?
Exam Questions
AQA May 23 Foundation Paper 1
This should be familiar as it is the symbol used on the Power Supply to indicate whether the supply is on or off.
If you are not certain about the answer then try the e-practical which will give you the answer.
You are given the numbers and the formula, so its 2 marks for being able to divide!
You are given the numbers and the formula, so its 2 marks for being able to divide again!
If you are not certain about the answer then try the e-practicals for the diode and the LED to which will give you the answers. The resistor is similar to the filament light but does not change its resistance with higher currents, so this component's graph is a straight line.
The big clue her is that it is not connected, so, the current must be zero but it is indicating a value.
This section is adapted from material developed by Dr Robert Lucas and is related to the book High School and Undergraduate Physics Practicals, published by CRC Press.